1. Technical Field
The present invention relates to a method and apparatus for a servo control system in a disk drive, and more particularly to a method and apparatus for avoiding repeatable runout errors in a disk drive.
2. Description of the Background Art
In a disk drive, such as a magnetic hard disk drive, a storage disk has a plurality of concentric tracks for storing information. The disk drive positions a transducer or read/write head over a desired track to read data from or write data to the disk. The movement of the head to a desired track is referred to as "seeking." Maintaining the head over the center of the desired track during read or write operations is referred to as track "following." These operations are controlled by a servo control system using prerecorded servo information either on a dedicated servo disk or on sectors interspersed among the data on a data disk. An example of a sectored servo disk drive architecture is provided in commonly owned U.S. Pat. No. 5,210,660.
Servo information in an embedded servo disk format typically includes a grey code or track identifier region comprising, for example, a cylinder or track number and sector number, enabling the servo electronics to seek to an appropriate track and data region. The servo information further comprises alignment information, such as a burst pattern used by the servo electronics to align the head with respect to the center of a desired track, e.g. when reading and writing data to the disk. During track following, the servo burst information sensed by the head is demodulated to generate sampled head position error signals indicating the deviation of the head from the track center. The position error signal is then further processed to produce a position signal. The servo feedback loop uses the position signal to generate a control signal to the actuator to move the head back to the track center.
Several factors cause the head to be off the track center during track following, and therefore affect the position signal. Certain position signal components have a low frequency, i.e. a frequency close to the frequency of the disk rotation, and repeat with disk rotation. For example, if the disk is not precisely centered over the axis of rotation of the spindle motor, the tracks will have an eccentric shape relative to the axis of rotation. The eccentric shape of the tracks causes a repeatable error at the same frequency as the disk rotation. Similarly, even if the disk is precisely mounted about the spindle motor axis, the spindle motor axis can wobble during rotation. The wobble will also generate repeatable errors, typically at the fundamental and second harmonic frequencies of the disk rotational frequency. Mechanical disturbances during servo writing are another potential cause of repeatable error, as is disk deformation. The problem of repeatable error is magnified in disk files which have multiple disks, because each disk and each disk surface will have its own unique repeatable error signature. For purposes of this disclosure, all of the preceding causes of repeatable error, and others generally understood in the art, will be referred to as repeatable runout.
Generally, a feedback control system controls the position of the head by generating an electrical control signal in response to the position signal. The control signal causes the head to move to a target position. However, the control signal is often less than ideal due to additional factors in the system. For example, repeatable runout is a disturbance that the control system cannot follow. Consequently, the deviation of the head from the target position may exceed an allowable value.
To accurately follow a track despite repeatable errors, a control system may use either the measured or predicted repeatable error to generate a signal which modifies the control signal to the actuator so that the head follows the true shape of the track center. One example of such a control system uses a digital filter having a high gain for particular frequency components. Control signals are input to the digital filter, which modifies the control signal to match the frequency characteristic of the repeatable error. In conventional disk drives, the filter is used to counteract read errors only after repeatable runout errors have been detected. Therefore, such errors are not prevented, e.g., when the disk drive is first turned on or when returning from sleep mode. In the state of the storage device referred to as "sleep mode", the power to the control system is turned off after a predetermined time interval. In either circumstance, the inability to filter out repeatable runout may prevent data retrieval, e.g. the retrieval of microcode stored on the disk and required for normal drive operations.